Electric induction furnace



3 .Sheets-Sheet 2 F. LINNHOFF sLscTRIc INDUCTION Fummc `Ftlsd July 22.

Fig 3.

Sept. 27, 1932.

3 Sheets-Sheet 3 Filed Ju'ly 22. 1929 Franz Linnhoff By Patented Sept.27, -1932 `UNITED STATES PATENT OFFICE FRANZ LINN'HOFF, OF E'BERSWALDE,GERMANY, ASSIGNOR TO AJAX ELECTROTHERMIC CORPORATION, OF AJAX PARK, NEWJERSEY, A CORPORATION 0F NEW JERSEY ELECTRIC INDUCTION FURNACEApplication led .Tuly 22, 1929, Serial No. 380,139, and in Germany July24,1928.

My invention relates to electric induction furnaces, more particularlyto coreless induction furnaces.

These furnaces have the disadvantage that a considerable part of themagnetic force lines does not flow through the melting bath, but takesits way through the air outside of the coil. This strong magneticleakage makes the power factor of the furnace worse and increases theoperating costs; moreover, it makes it impossible to provide the furnacewith an iron supporting frame because such a frame would be heatedhighly by the eddy currents induced in it by the leakage flux.

It has been tried to prevent the leakage by arranging a yoke oflaminated iron round the coil. These yokes consist either in a cylindersurrounding'the coil or in columns of laminated iron extending parallelto the axis of the coil. Experience has shown that even with suchfurnaces provided with yokes a considerable leakage takes place. Thereason for this leakage lies in the fact, that, with both yoke types,the magnetic force lines which follow the yoke change their directionabruptly at the top of the yoke between it and the bath.Y As a resultconsiderable leakage takes place in this part of the magnetic circuitgiving a large leakage field.

The object of my invention is to eliminate the magnetic leakage entirelyand to increase the power factor of the furnace.

Figure l is an approximately central vertical section through thestructure of Figure 2 taken upon line l-l of that figure.

Figures 2 and l are top plan views of variant forms of my furnace.

Figure 3 is a section corresponding in position with Figure l but takenupon line 3-3' of Figure 4.

Figure 5 is a central vertical section showing a further modified form.

Figure 6 is a fragment of Figure l, modified to show a somewhatdifferent form.

My invention consists in a coreless induction furnace with a yokearranged round the coil, the yoke being formed so that the magneticforce lines can pass from the yoke to the melting bath and inverselywithout considerable changement of their direction. With thisarrangement any leakage is avoided, since the path of the magnetic forcelines in the air and in the melting bath represents the short` est waybetween both yoke ends so that they are not inclined to leave it.

By the concentration of the flux on the 'melting bath the power factoris increased melting bath. The ends of thc yoke may ap-I proach thecrucible from the side,'but in order to utilize the magnetic iiuXentirely it is particularly favourable to lead the ends of the yokeabove and below the melting bath. In furnaces which are charged fromabove, the yoke can be subdivided in such a manner that its upper partbe removable together with the cover of the furnace.

The yoke may consist in several arms of laminated iron arranged roundthe coil, as usual. In order to attain a good distribution of themagnetic fiux, the single arms can be connected one to another at one orboth ends by means of rings of magnetic material.

The iron of the yoke can be laminated in radial or tangential planes.For avoiding a too strong heating of the innerparts of the yoke by theleakageiiux fiowmcr in these parts, they may be provided with horizontalslits S, Figure 6, of Convenient depth. These slits increase themagnetic. resistance of the inner parts of the `yoke .and thereby causethe'flux to distribute more uniformly in the inner and outer part of theyoke.

The slits S are shown as extending hori- Zontally outwardly from theinside of the vertical leg of the yoke, extending part way only throughthe leg.

In the accompanying drawings there are illustrated some embodiments ofthe innention.

Figs.. l and 2 show an induction furnace 10c with the crucible 1,-thecover 2 and the suporting ring 3. The Crucible is surrounded y theinduction coil 4 arranged between rings 5 and 6. The yoke'consists ofseveral arms 7 distributed round the coil, the arms being C-shaped andextending above and below the Crucible. The furnace is supported by-aframe 9 and can be tilted over the 'axis 10. The cover and the upperpart :of the yoke can be removed by means of the loo 13 fastened to theplate l2 which is insufated from the yoke by a disk 11.

Figs. 3 and 4 show a furnace witha yoke consisting of arms 7 which areconnected one to another at their upper end lower ends by rings 15.

Fig. 5 shows a furnace with a yoke which does not interfere with thecover since its ends are inclined toward the furnace approaching it fromthe side.

Having now particularly described and:

ascertained the nature of my invention and in what manner the same is tobe performed, what I claim is:

l. In an electric induction fur-nace, in combination, a Crucible, aninduction coil surrounding said Crucible and a C-shaped yoke surroundingsaid coil, the inner part of the yoke being provided with horizontalslits and being removable bodily at the top from the side members.

2. A pot-type of induction furnace including a coil surrounding thefurnace, ring pole pieces respectively across the topand bottom ofthemetal of the furnace and laminated connections between the pole pieces.

3. A pot-type of induction furnace including a. coil surrounding thefurnace, rinU pole pieces respectively across the top and bottom of themetal of the furnace and laminated connections between the polevpieces,-`

the upper pole piece being removable.

4. A pot-type of furnace having a removable lid, an induction coilsurrounding the furnace and a'niagnetic pole piece across the bottom ofthe furnace, a removable cover upon the top of the furnace, uprightlaminated magnetic conductors spaced around the sides of the furnace andconnecting at their lower ends with the bottom pole pieces and a ringpole piece above the cover directly above the metal of the furnace andhaving laminated extensions to the upright members.

In testimony whereof I affix my signature.

FRANZ LI'NNHOFF.

